This invention relates generally to nuclear reactors and, particularly, to reactors having fluid pressure operated control rod mechanisms.
The core of a modern pressurized water cooled reactor of 1000 MWE output contains approximately 40,000 individual fuel rods, each having two welded end plugs. The total length of the rods is nearly 100 miles. With the high number of welds and large clad surface area, there is a probability that one or more fuel rods will develop defects during operation even though the highest grade of quality control is maintained during the manufacture of the rods. The defects probably will be in the form of pin holes or cracks in welds or cladding material. In any case, it permits escape of some of the fission products into the coolant stream and causes a rise in radioactivity in the entire coolant system. (The most abundant of the fission products are Kr88, Rb88, I131, I133, Xe133, Xe135 and Cs137). A certain amount of fission product leakage can be tolerated without causing too much of a problem, since the level of radioactivity can be limited by continuous removal of the fission products with available systems. For example, the Xenon gases are removed by gas stripping techniques in the volume control tank and the gas decay tank, while the others are removed in the demineralizers. However, if the leakage of fission products into the coolant exceeds the capacity of these systems, the general level of radioactivity gradually increases until it exceeds permissible limits and it becomes necessary to shut down the reactor or, at least, to continue operation at reduced power.
After the reactor is shut down, the task of locating and removing the defective fuel assembly still remains. In prior nuclear power plants, means are available for determining during operation only that a leaky fuel assembly is present in the core and, at best, in which quadrant it is located. Pin-pointing of the fuel assembly requires removal of all fuel assemblies in turn to a special inspection chamber for testing. Since the production of fission products ceases at shutdown and the possibility exists that a crack may seal itself when the temperature is reduced, the detection of a defective fuel assembly becomes a very difficult and intricate task which may require a month or more of reactor shutdown time.
Therefore, it is desirable to provide a system for determining the exact location of a defective fuel assembly during normal reactor operation to permit speedy removal of the assembly before conditions become intolerable, or preferably during a scheduled shutdown. The latter becomes a real possibility if a "Rapid Refueling" system is adapted since scheduled refuelings take place at much shorter intervals than with conventional reactors. Also, it is desirable to simplify the structure of the upper internals of a reactor to facilitate testing for a defective fuel assembly.